Patient-ventilator synchronization using dual phase sensors

ABSTRACT

An improved ventilator which delivers ventilatory support that is synchronized with the phase of the patients respiratory efforts and guarantees a targeted minimum ventilation. Improved synchronization is achieved through an instantaneous respiratory phase determination process based upon measured respiratory airflow as well as measured respiratory effort using an effort sensor accessory, preferably a suprasternal notch sensor. The ventilator processes a respiratory airflow signal, a respiratory effort signal and their respective rates of change to determine a phase using standard fuzzy logic methods. A calculated pressure amplitude is adjusted based upon the calculated phase and a smooth pressure waveform template to deliver synchronized ventilation.

This application claims the priority filing date of U.S. ProvisionalApplication Ser. No. 60/154,196 filed on Sep. 15, 1999.

FIELD OF THE INVENTION

This invention relates to a method and device for providing ventilatoryassistance to a patient. More specifically, the invention involves animproved method and device that provides ventilation in phase with apatient's respiratory efforts through the use of a respiratory effortsensor.

BACKGROUND OF THE INVENTION

Devices for providing mechanical ventilation to assist patientrespiration are well known. Such ventilator devices have been used tohelp patients with such ailments as severe lung disease, chest walldisease, neuromuscular disease and other diseases of respiratorycontrol. Generally, a ventilator provides air or oxygen-enriched air toa patient at pressures that are higher during inspiration and lowerduring expiration.

Several types of ventilator devices exist. These types include bi-levelventilators, proportional assist ventilators and servo-controlledventilators. Each type of ventilator utilizes different methods forassisting with patient respiration and achieves different goals.

Bi-level ventilators provide the simplest level of support. Theseventilators supply a mask pressure P(t) which is higher by an amplitudeA from an initial pressure P₀ when respiratory airflow f(t) isinspiratory, f(t)>0, than when respiratory airflow is expiratory,f(t)≦0.

-   -   P(t)=P₀+A f(t)>0 (inspiration)    -   P(t)=P₀ otherwise (expiration)        Thus, these devices supply a fixed degree of support A. However,        they do not guarantee any particular ventilation when, for        example, the patient's efforts are inadequate.

Proportional assist ventilators represent an attempt to provide supportmore closely in phase with the patient's respiratory efforts.Proportional assist ventilators provide mask pressure as follows:

-   -   P(t)=P₀+R f(t)+E_(LC)∫f(t)dt f(t)>0 (inspiration)    -   P(t)=P₀+R f(t) otherwise (expiration)        where R is a substantial fraction of the patient's airway        resistance, and E_(LC) is a substantial fraction of the        patient's lung plus chest wall elastance. So long as there is no        leak, this provides support much more closely in phase with a        patient's respiratory efforts. However, again, in the case of a        patient's efforts being inadequate, for example, due to reduced        chemoreflex control of breathing during sleep, there is no        guaranteed minimum ventilation.

One method of ensuring an adequate degree of ventilatory support is touse a servo ventilator, which adjusts the degree of support A toservo-control instantaneous ventilation V(t) to equal a targetventilation V_(TGT):

-   -   P(t)=P₀+A f(t)>0, or time since the start of the last        inhalation>T_(max)    -   P(t)=0 otherwise

where:

-   -   A=G f(V(t)−VTGT) dt    -   V(t)=0.5 abs (f(t))    -   A_(MIN)<A<A_(MAX).        In this ventilator, V(t) is one half the absolute value of the        respiratory airflow, G is the gain of the integral        servo-controller, a value of 0.3 cmH₂O per L/min error in        ventilation per second being suitable, and A_(MIN) and A_(MAX)        are limits set on the degree of support A for comfort and        safety, 0.0 and 20.0 cmH₂O being generally suitable.        Unfortunately, while this method achieves a guaranteed minimum        ventilation, there is little attempt to keep the support        precisely in phase with the patient's own respiratory efforts.        Rather, the system merely makes a step change in pressure at the        start and end of inspiration.

In a more advanced servo-controlled ventilator, both guaranteedventilation and phase synchronization is achieved. This apparatus is thesubject of the commonly owned International Patent Application entitled“Assisted Ventilation to Match Patient Respiratory Need,” InternationalPublication Number WO 98/12965 (hereinafter referred to as “AutoVPAP”).The AutoVPAP apparatus provides an instantaneous mask pressure P(t)based upon a substantial fraction of the patient's airway resistance R,respiratory airflow f(t), an amplitude A, and an estimation of thepatient's instantaneous respiratory phase Φ as applied to a smoothpressure waveform template π(Φ) as follows:

-   -   P(t)=P₀+R f(t)+Aπ(Φ) for all f(t) (inspiration and expiration)

where:

-   -   A=G∫(V(t)−V_(TGT))dt    -   V(t)=0.5 abs (f(t))    -   A_(MIN)<A<A_(MAX)        In estimating the respiratory phase, the AutoVPAP apparatus uses        a respiratory airflow signal and its derivative as input data        for a set of fuzzy logic rules that are associated with        particular phases of respiration. Using the results of the        evaluations of the rules, a single phase value is derived and        used as the instantaneous respiratory phase. Thus, the degree of        ventilatory support is varied in phase with the patient's        respiration. Moreover, as the calculation of A is based upon a        target ventilation V_(TGT), a guaranteed level of ventilation is        provided.

However, in this AutoVPAP system, room for improvement exists in thephase determination due to the problem of leak. Mask and/or mouth leakis ubiquitous during noninvasive ventilatory support using a mask, andis particularly problematical during sleep. Leak causes mis-measurementof the respiratory airflow, and therefore can severely interfere withpatient-machine synchronization.

BRIEF DESCRIPTION OF THE INVENTION

An objective of the present invention is to provide a method andapparatus to supply ventilatory assistance precisely in phase with apatient's spontaneous ventilatory efforts provided these efforts areadequate to maintain a specified target ventilation.

A further objective of the present invention is to provide a method andapparatus to guarantee at least a specified target ventilation, even inthe event that the patient's efforts become irregular or cease.

Additional objectives will be apparent from the description of theinvention as contained herein.

In its broadest aspects, the present invention involves an improvedventilator which delivers ventilatory support that is synchronized withthe phase of a patient's respiratory efforts and guarantees a targetedminimum ventilation. The device provides ventilatory support to apatient based in part upon a calculated instantaneous mask pressureusing a general method similar to that of the previously describedAutoVPAP system. However, the respiratory phase determination isimproved by using data representing both respiratory airflow andrespiratory effort.

More specifically, in estimating the respiratory phase, the deviceutilizes a respiratory airflow signal and preferably its rate of change.The degree of membership of the respiratory airflow signal in each ofthe fuzzy sets zero, positive, large positive, negative, and largenegative is calculated using suitable membership functions. Similarly,the degree of membership of the derivative of the respiratory airflowsignal in each of the fuzzy sets steady, increasing, increasing fast,decreasing and decreasing fast is calculated using suitable membershipfunctions. The degrees of membership in these classes are used in a setof fuzzy logic inference rules. Each fuzzy inference rule is associatedwith a particular phase of respiration.

Although many variations are possible, in the preferred embodiment, theinference rules relating to respiratory airflow are as follows:

-   -   1. If the airflow is zero and increasing fast, then the phase is        0 revolutions.    -   2. If the airflow is large positive and steady, then the phase        is 0.25 revolutions.    -   3. If the airflow is zero and falling fast, then the phase is        0.5 revolutions.    -   4. If the airflow is large negative and steady, then the phase        is 0.75 revolutions.    -   5. If the airflow is zero and steady and the 5-second low-pass        filtered absolute value of the respiratory airflow is large,        then the phase is 0.9 revolutions.    -   6. If the airflow is positive and the phase is expiratory, then        the phase is 0.1 revolutions.    -   7. If the airflow is negative and the phase is inspiratory, then        the phase is 0.6 revolutions.    -   8. If the 5-second low-pass filtered absolute value of the        respiratory airflow is small, then the phase in the respiratory        cycle is increasing at a fixed rate equal to the patient's        expected respiratory rate.    -   9. If the 5-second low-pass filtered absolute value of the        respiratory airflow is large, then the phase in the respiratory        cycle is increasing at a steady rate equal to the existing rate        of change of phase, low-pass filtered with a time constant of 20        seconds.

In the invention, the device combines additional fuzzy inference rulesbased upon an effort signal from an effort sensor. The effort sensor isnot dependent on measured airflow and as such is immune to errorsassociated with leak. The effort signal and preferably its rate ofchange are used as input values for membership functions for theadditional fuzzy inference rules that are also associated withparticular phases. The membership functions are used to calculate thedegree of membership of the effort signal in each of the fuzzy setszero, medium and large, and the degree of membership of its derivativein each of the fuzzy sets increasing moderately, increasing fast,decreasing moderately and decreasing fast.

In the preferred embodiment, the inference rules relating to respiratoryeffort are as follows:

-   -   10. If the effort signal is zero and increasing fast, then the        phase is 0 revolutions.    -   11. If the effort signal is medium and increasing moderately,        then the phase is 0.2 revolutions.    -   12. If the effort signal is large and decreasing fast, then the        phase is 0.5 revolutions.    -   13. If the effort signal is medium and decreasing moderately,        then the phase is 0.7 revolutions.        In general, the phase values need not be these exact values, but        can approximate them.

In a process similar to that of the AutoVPAP system, all of the fuzzyinference rules are evaluated and a single phase value is derived andtaken as the instantaneous respiratory phase of the patient. This allowsthe degree of support provided by the device to remain in phase with thepatient's respiration.

In the preferred embodiment, the effort sensor is a suprastemal notchsensor. However, alternative embodiments of the invention may utilizeother effort sensors including, for example, an esophageal pressuresensor or an electromyograph.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the components of a servo-controlled ventilator with aneffort sensor; and

FIG. 2 depicts the steps involved in determining the delivered pressurelevel, including use of the effort signal.

DETAILED DESCRIPTION OF THE INVENTION

A servo-controlled ventilator useful for accomplishing the presentinvention is shown in FIG. 1. A blower 10 supplies air under pressurevia delivery tube 12 to a mask 11. Exhaust gas is vented via exhaust 13.Mask flow is measured using pneumotachograph 14 and differentialpressure transducer 15 to derive flow signal f(t). Mask pressure ismeasured at pressure tap 17 using pressure transducer 18. Respiratoryeffort is measured by an effort sensor 22 to yield an effort signalE_(ss)(t). Flow, effort and pressure signals are sent to microcontroller16 which implements the processing shown in FIG. 2 to derive a pressurerequest signal P(t). The actual measured pressure and pressure requestsignal P(t) are fed to motor servo 19 which controls blower motor 20 toproduce the desired instantaneous mask pressure. An example of this typeof ventilator, without an effort sensor 22, is the subject ofInternational Publication No. WO 98/12965, which is also disclosed inrelated U.S. application Ser. No. 08/935,785. An additional example isdisclosed in International Publication No. WO 99/61088, which is alsocontained in related U.S. application Ser. No. 09/316,432. The foregoingU.S. applications are hereby incorporated by reference.

In the preferred embodiment of the present invention, a suprasternalnotch sensor is used as the effort sensor 22 to generate the effortsignal. The sensor is more fully described in a commonly owned patentapplication entitled “Measurement of Respiratory Effort Using aSuprasternal Sensor,” application Ser. No. 09/396,031 filed on Sep. 15,1999. The suprasternal notch sensor measures changes in the suprasternalnotch. Increasing inspiratory efforts cause the skin of the suprasternalnotch to retract. An optical sensor generates an electrical signal thatis an increasing function of inspiratory effort derived by measuringchanges in the depth of the skin of the suprasternal notch. In a furthercommonly owned patent application filed on Sep. 15, 1999, entitled“Ventilatory Assistance Using an External Effort Sensor,” applicationSer. No. 09/396,032, the effort signal is used to trigger a bilevelventilator. Both applications are hereby incorporated by reference.

With reference to FIG. 2, using the aforementioned system of FIG. 1,several steps are used to derive the pressure request signal P(t). Insteps 30 and 32, an instantaneous airflow signal f(t) and an effortsignal E_(ss)(t) are generated. In step 34, labeled Fuzzy PhaseCalculation, the system calculates the phase Φ in the respiratory cycleas a continuous variable using a set of fuzzy inference rules. The fuzzyinference rules are based upon both respiratory airflow and respiratoryeffort.

The preferred rules with respect to respiratory airflow are thosedescribed above, rules 1-9, although many other inference rules can bedeveloped based upon respiratory airflow. The fuzzy extents, or thedegrees of truth, to which the airflow is in fuzzy sets zero, positive,large positive, negative, and large negative, and the 5-second low-passfiltered absolute value is small and large, are determined with suitablemembership functions using the measured respiratory airflow f(t).Similarly, the fuzzy extents to which the airflow is in fuzzy setssteady, increasing fast and decreasing fast are determined with suitablemembership functions using the change in respiratory airflow df(t)/dt.The fuzzy extents to which the phase is a member of the fuzzy setsexpiratory and inspiratory is determined by membership functions using apreviously calculated instantaneous phase. This approach usingrespiratory airflow to determine phase is that disclosed inInternational Publication Nos. WO 98112965 and WO 99/61088.

Rules 1-4 estimate the phase directly from the instantaneous respiratoryairflow. Rule 5 permits an expiratory pause, whose length may be long ifthe patient has recently been breathing adequately, and short or zero ifthe patient is not breathing. Rules 6-7 provide for quickresynchronization in the event that the patient breathes irregularly.Rule 8 provides the equivalent of a timed backup, in which, to theextent that the patient has stopped breathing or is not adequatelybreathing, the ventilator will cycle at a suitable fixed rate. Rule 9provides that to the extent that the patient is breathing adequately,the ventilator will tend to track the patient's recent averagerespiratory rate. This is particularly appropriate for patients withcardiac failure and Cheyne-Stokes breathing, whose respiratory ratestend to be extremely steady despite rhythmic changes in amplitude.

An effect of the changing degree of activation of rules 8 and 9 is that,to the fuzzy extent that the instantaneous ventilation equals or exceedsthe target ventilation, ventilatory support will be provided in phasewith the patient's own respiratory efforts, and to the extent that theinstantaneous ventilation is less than the target ventilation,ventilatory support will be provided at a pre-set rate.

In an elaboration of this embodiment, the weighting of rules 1-6 can bemade proportional to the fuzzy extent that the instantaneous ventilationis large compared with the target ventilation, thereby reinforcing thebehavior described in the previous paragraph.

In a further elaboration, the weighting of rules 1-6 and also of rule 9can be made smaller and the weighting of rule 8 can be made larger, ifthe leak is large or if there is a sudden change in the leak. In thisway, to the extent that the respiratory airflow signal is of highquality, ventilatory support will be provided as described in thepreceding paragraphs, but to the extent that the respiratory airflowsignal is of poor quality and it is difficult to synchronize reliablywith the patient's efforts, or to know if the patient's efforts areadequate, ventilatory support will be provided in an orderly manner at apredetermined fixed rate.

As previously mentioned, the Fuzzy Phase Calculation of step 34 alsoinvolves fuzzy inference rules relating to respiratory effort. To thisend, the invention takes the effort signal E_(ss)(t) from the effortsensor 22 and processes additional fuzzy inference rules.

The general method for developing and using these additional fuzzyinference rules for the effort signal is the same as the methoddescribed in International Publication Nos. WO 98/12965 and WO 99/61088.Generally, various features, such as the point of start of inspiration,are identified on a graph of effort versus phase, and for each phase,corresponding fuzzy rules are developed. For example, a suitable rulefor the point “start of inspiration” could be “effort signal is smalland the second derivative of the effort signal with respect to time islarge positive.” Membership functions, would cause that rule to bemaximally activated at or near the start of inspiration. Preferably, theexact phase at the moment of maximal activation should be determinedempirically. In the current example, the maximum activation will be at aphase shortly after the actual moment of start of inspiration, say 0.05revolutions, and this is the best phase to associate with the rule. Themore features that are identified and assigned a rule and a phase, thesmoother will be the resultant determination of phase.

The illustrative additional fuzzy inference rules relating to the effortsignal E_(ss)(t) from the suprasternal effort sensor and the rate ofchange in the signal, dE_(ss)(t)/dt, are rules 10-13 provided above.

The fuzzy extents to which the effort signal is in fuzzy sets zero,medium and large are calculated with appropriate membership functionsusing E_(ss)(t). Similarly, the fuzzy extents to which the effort signalis in fuzzy sets increasing moderately, increasing fast, decreasingmoderately or decreasing fast are determined with appropriate membershipfunctions using the rate of change of the effort signal, dE_(ss)(t)/dt.Preferably, the effort signal is normalized for amplitude prior tocalculation of degrees of membership, for example, by dividing by theamplitude of the effort signal calculated over a long period comparedwith a breath, for example, 10-30 seconds.

Continuing with FIG. 2 and the Fuzzy Phase Calculation of step 34, eachof the rules in the combined set of fuzzy inference rules is evaluatedto determine a degree of activation G(n) by using a standard fuzzyinference method. For example, with respect to rule 12, using one suchmethod assuming a unit weighting of rules, if (a) the degree of truthfor the membership function “the effort signal is large” evaluates to0.6 and (b) the degree of truth for the membership function “the effortsignal is decreasing fast” evaluates to 0.4, and a fuzzy logic “AND”operator is applied, then the degree of activation for G(12) would be0.4.

Additionally, each of the 13 fuzzy inference rules associates aparticular rule with a particular phase Φ(n). For example, as shownabove, rule 12 is associated with Φ(12)=0.5 revolutions. Then, using thedegree of activation G(n) for each Φ(n), a single value representing theinstantaneous respiratory phase Φ is calculated in a defuzzificationstep using the formula:Φ=arctan{Σ[G(n) sin Φ(n)]/Σ[G(n) cos Φ(n)]}.The phase Φ is then used in step 36 of FIG. 2 to derive a value from thesmooth pressure waveform template π(Φ).

Step 38, labeled “Amplitude Calculation,” involves a calculation of aninstantaneous amplitude of pressure support A, chosen to servo-controlinstantaneous ventilation to, on average, equal a target V_(TGT), by thefollowing formula:A=G∫(V(t)−V _(TGT))dtV(t)=0.5 abs(f(t))A_(MIN)<A<A_(MAX)

Finally, in the “Pressure Calculation” step 40, the system calculatesthe desired degree of ventilatory support P(t) from the calculatedamplitude A, a substantial fraction of the patient's airway resistanceR, respiratory airflow f(t), and the smooth pressure waveform templateπ(φ):P(t)=P ₀ +Rf(t)+Aπ(Φ)The Amplitude Calculation and Pressure Calculation steps as justdescribed are comparable to the steps in International Publication Nos.WO 98/12965 and WO 99/61088.

In the preferred embodiment of the invention, the effort sensor is anaccessory that can be added or removed at will from a ventilator such asan AutoVPAP system in order to achieve improved synchronization with thepatient. If the effort sensor falls off the patient, fails, iselectrically unplugged or is otherwise removed, then rules (10) to (13)will have no effect on the phase determination or, in other words, therules will have no degree of activation. In this event, the device willbehave as a simple servo-controlled ventilator such as the AutoVPAPdevice. The patient will continue to be ventilated, with ventilatorysupport provided approximately in phase with respiratory airflow andtherefore approximately in phase with respiratory effort. Thus, thedegree of support will be sufficient to guarantee that instantaneousventilation on average equals or exceeds the target ventilation, but theprecise timing information and improved immunity to the effects of leakswill be lost.

While the described embodiment of the present invention makes use of asuprasternal effort sensor, effort signals from other sensors may alsobe utilized. Other forms of effort signal include, for example, aneffort signal derived from esophageal pressure generated by a pressuretransducer implanted in the chest that sends a signal via telemetry.Alternatively, an electromyogram signal from an electromyograph could beused without requiring any modification to the invention. Moreover,multiple effort sensors can be utilized simply with the addition ofextra fuzzy inference rules relating to the additional effort sensors.

Although the invention has been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of an application of the principles of the invention. Forexample, one embodiment of the invention might utilize a set of rules todetermine phase in which some of the rules determine phase based solelyupon the magnitude of the respiratory airflow and the rate of change ofan effort signal. Alternatively, some rules may determine phase basedupon the magnitude of the effort signal and the rate of change ofrespiratory airflow. Numerous modifications, in addition to theillustrative embodiments of the invention discussed herein may be madeand other arrangements may be devised without departing from the spiritand scope of the invention.

1-40. (canceled)
 41. A method of providing ventilatory support to apatient comprising the steps of: providing apparatus to deliverventilatory support to the patient, the apparatus also measuring bothrespiratory airflow and respiratory effort; calculating a desiredpressure value based at least in part on the measured respiratoryairflow and respiratory effort; and delivering ventilation to saidpatient in accordance with said desired pressure value.
 42. The methodof claim 41 wherein said respiratory effort is measured by a sensorselected from a group of effort sensors that are independent of a leakin airflow that may affect respiratory airflow measurement including:(a) a suprasternal notch sensor; (b) an esophageal pressure effortsensor; and (c) an electromyograph.
 43. The method of claim 41 whereinthe desired pressure value is also based at least in part on theinstantaneous phase of respiration of the patient determined byevaluating fuzzy inference rules relating to a signal from saidrespiratory effort sensor.
 44. The method of claim 41 wherein thedesired pressure value is also based at least in part on theinstantaneous phase of respiration of the patient determined byevaluating fuzzy inference rules relating to the rate of change of asignal from said respiratory effort sensor.
 45. The method of claim 41wherein the desired pressure value is also based at least in part on theinstantaneous phase of respiration of the patient determined byevaluating fuzzy inference rules relating to the measured respiratoryairflow.
 46. The method of claim 41 wherein the desired pressure valueis also based at least in part on the instantaneous phase of respirationof the patient determined by evaluating fuzzy inference rules relatingto the rate of change of the measured respiratory airflow.
 47. Themethod of claim 41 wherein the desired pressure value is varied betweena non-zero minimum value and a maximum value.
 48. The method of claim 47wherein the ventilation delivered to the patient is calculated and thedesired pressure value is calculated at least in part as a function ofthe difference between the calculated patient ventilation and a targetvalue.
 49. An apparatus for providing ventilatory support to a patientcomprising: at least one sensor to generate a respiratory effort signal;at least one sensor to generate a respiratory airflow signal; aprocessor for determining a pressure request signal at least in part asa function of the effort signal and the airflow signal; and a blower toprovide pressurized air to said patient in accordance with said pressurerequest signal.
 50. The apparatus of claim 49 wherein said respiratoryeffort signal is generated by a sensor selected from a group of effortsensors that are independent of a leak in airflow that may affect saidrespiratory airflow signal including: (a) a suprasternal notch sensor;(b) an esophageal pressure effort sensor; and (c) an electromyograph.51. The apparatus of claim 49 wherein the pressure request signal isalso determined at least in part as a function of the instantaneousphase of respiration of the patient determined by evaluating fuzzyinference rules relating to said respiratory effort signal.
 52. Theapparatus of claim 49 wherein the pressure request signal is alsodetermined at least in part as a function of the instantaneous phase ofrespiration of the patient determined by evaluating fuzzy inferencerules relating to the rate of change of said respiratory effort signal.53. The apparatus of claim 49 wherein the pressure request signal isalso determined at least in part as a function of the instantaneousphase of respiration of the patient determined by evaluating fuzzyinference rules relating to said respiratory airflow signal.
 54. Theapparatus of claim 49 wherein the pressure request signal is alsodetermined at least in part as a function of the instantaneous phase ofrespiration of the patient determined by evaluating fuzzy inferencerules relating to the rate of change of said respiratory airflow signal.55. The apparatus of claim 49 wherein the pressure request signal isvaried between a non-zero minimum value and a maximum value.
 56. Theapparatus of claim 55 wherein the ventilation delivered to the patientis calculated and the pressure request signal is calculated at least inpart as a function of the difference between the calculated patientventilation and a target value.